1. Field of the Invention
The present invention relates generally to wireless communication systems and more particularly to a system for enabling mixed mode wireless communications by forward and backward compatible systems.
2. Description of the Related Art
Wireless communications systems enable various types of communications. One type of wireless communication between a single transmitter and a single receiver is known as a single-output-single-input (SISO) communication. The transmitter includes one antenna for transmitting radiofrequency (RF) signals, which are received by one or more antennae of the receiver. When the receiver includes two or more antennae, the receiver selects one of antennae to receive the incoming RF signals. Another type of wireless communications is a multiple-input-multiple-output (MIMO) communication. In a MIMO wireless communication, the transmitter and the receiver each includes multiple paths. In such a communication, the transmitter parallel processes data using a spatial and time encoding function to produce two or more streams of data. The transmitter includes multiple transmission paths to convert each stream of data into multiple RF signals. The receiver receives the multiple RF signals via multiple receiver paths that recapture the streams of data utilizing a spatial and time decoding function. The recaptured streams of data are combined and subsequently processed to recover the original data.
Different wireless devices in a wireless communication system may be compliant with different standards or different variations of the same standard. For example, IEEE™ 802.12a, an extension of the IEEE™ 802.11 standard, provides up to 54 Mbps in the 5 GHz band. IEEE™ 802.12c, another extension of the 802.11 standard, provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. IEEE™ 802.11g, another extension of the 802.11 standard, provides 20+ Mbps in the 2.4 GHz band. IEEE™ 802.11n, a new extension of 802.11, is being developed to address, among other things, higher throughput and compatibility issues. An 802.12a compliant communications device may reside in the same WLAN as a device that is compliant with another 802.11 standard. When devices that are compliant with multiple versions of the 802.11 standard are in the same WLAN, the devices that are compliant with older versions are considered to be legacy devices. To ensure backward compatibility with legacy devices, specific mechanisms must be employed to insure that the legacy devices know when a device that is compliant with a newer version of the standard is using a wireless channel to avoid a collision. New implementations of wireless communication protocol enable higher speed throughput, while also enabling legacy devices which might be only compliant with 802.12a or 802.11g to communicate in systems which are operating at higher speeds.
To address the compliance issue, current communications systems support mixed-mode frames for two or more transmit antennae. The mixed mode frame includes information that can be used by legacy devices and information that is specific to non-legacy devices. Specifically, the mixed mode frame includes a preamble that includes a backward compatible portion and a greenfield portion. The backward compatible portion is used by a legacy device to determine how long to stay off the channel due to transmission of the greenfield portion which is ignored by the legacy device but used by a non-legacy device.
There is a need, however, for packets or frames to be transmitted to devices at higher rates, where possible, and also be compliant with legacy devices.